Why Vacuum Parts Are Made From Oxygen Free Copper
Updated September 26, 2026
Vacuum work is the one corner of copper machining where the grade is not negotiable and the machining rules are stricter than the tolerances suggest. If you are buying chamber parts, gaskets or feedthroughs, this is the background you need to write a drawing that a shop can actually deliver against.
The two reasons OFE is specified
Hydrogen embrittlement. Electrolytic tough pitch copper, C110, contains a small amount of oxygen present as copper oxide. Heat that copper in a hydrogen atmosphere and the hydrogen diffuses in, reacts with the oxide and forms steam trapped inside the metal. The result is internal porosity and cracking, and the part fails in a way that inspection of the raw material would never have predicted.
Oxygen free copper, C10100 or C101, has the oxygen removed, so there is nothing for the hydrogen to react with. This is why every hydrogen furnace brazed component and every part that will be baked in a reducing atmosphere is specified as OFE.
Outgassing and inclusions. Ultra high vacuum systems are limited by what comes out of their own walls. Oxide inclusions and voids in the metal hold gas and release it slowly. The cleaner and more uniform the material, the better the ultimate pressure the system reaches.
The alloy data behind both points is published by the Copper Development Association.
Where it shows up
- Vacuum chamber bodies and flanges
- Copper gaskets for knife edge flange seals
- Feedthrough bodies and conductors
- Cryogenic components, where copper’s thermal conductivity at low temperature is also wanted
- RF cavities and accelerator components
- Sinker EDM electrodes, for the same structural uniformity reason
The machining rules that matter more than the tolerances
A vacuum part can be dimensionally perfect and still be useless. These are the requirements that a shop unfamiliar with vacuum work will miss.
No trapped volumes. Every blind tapped hole inside the vacuum envelope is a gas pocket once a bolt goes into it. Either vent the hole through to a pumped volume, or specify vented screws. Mark this on the drawing, because a shop cannot guess where your vacuum boundary is.
No burrs, anywhere. A burr is surface area and a place for contamination to sit. On sealing surfaces it is a leak path.
Surface finish on sealing faces. Knife edge seals cut into the gasket, and the gasket surface has to be clean and free of radial scratches. A circumferential scratch is usually tolerable. A radial one is a leak path.
No free-machining grades and no substitutions. The additions in C145 and C147 are contaminants in this context. This is the part of vacuum machining that costs money: you are cutting pure copper with all the burr and smear behaviour that entails, and you cannot swap the grade to make it easier. See why copper is difficult to machine.
Cutting fluid choice. Sulphur bearing and chlorinated cutting fluids leave residues that are difficult to remove and unwelcome in a vacuum system. Shops that do this work know which fluids they can use. Ask.
Cleaning and handling. Degreasing, often ultrasonic cleaning, sometimes electropolishing, then handling with gloves and packing in clean bags. A part that is machined perfectly and then handled with bare hands has fingerprints in it, and fingerprints outgas.
Bakeout compatibility. If the system gets baked, every material in the part has to survive that temperature, including any plating, any joint and any packaging material that touches it.
What to put on a vacuum RFQ
- Grade stated explicitly as C10100 or C101 oxygen free, and a note that no substitution is acceptable
- Material certificate required, if it is
- The vacuum boundary marked on the drawing
- Blind holes identified, with vent requirement or vented screw callout
- Sealing surfaces identified with finish requirement and scratch direction note
- Cleaning specification and final packaging requirement
- Whether the part will be brazed, and in what atmosphere
- Bakeout temperature the finished assembly must survive
- Leak test requirement, criterion and who performs it
That list is longer than a normal copper RFQ, and it is the reason vacuum parts cost more than their geometry suggests. A shop that receives all of it can quote properly. A shop that receives a model and the word “copper” will quote a part that fails.
Cost expectations
Set them realistically. You are machining the hardest copper grade to cut, with no grade substitution available, to a surface standard driven by sealing rather than by fit, followed by cleaning and controlled packaging. That is several operations a normal machined part does not have.
The saving available to you is in geometry rather than material. Generous internal radii, fewer blind holes, sealing surfaces only where they are needed and a realistic tolerance everywhere else. Design rules for machined copper parts applies here more than anywhere.
Frequently asked questions
Can I use C110 instead of C101 in a vacuum system?
For a rough vacuum component that is never brazed or baked, sometimes. For ultra high vacuum, for anything brazed in a hydrogen furnace and for sealing surfaces, the oxygen free grade is specified for physical reasons and substituting it is not a cost saving.
Why can't I use tellurium copper for vacuum parts?
The additions that make it machine well are exactly what a vacuum engineer does not want in the system. Free-machining grades are not used for ultra high vacuum components, which is why these parts are expensive to cut.
What is a virtual leak?
A trapped pocket of gas inside the vacuum envelope, typically under a bolt in a blind tapped hole. It bleeds out slowly and behaves like a leak that no leak detector can find outside the chamber. Vent holes and vented screws prevent it.
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